In this study, monkeys will be used to study vestibular control of locomotion. Compensatory and orienting components of the vestibulo-ocular and vestibular-cervical reflexes arising in the otolith organs and semicircular canals that are used to stabilize posture and gaze during natural quadrupedal and bipedal locomotion will be characterized. Based on preliminary data, it is postulated that there is similar vertical translation of the body and head during quadrupedal locomotion in monkeys as during bipedal walking in humans and that these translations activate linear vestibulo-collic and vestibulo-ocular reflexes (IVCR & lVOR) to stabilize head position in pitch with subsequent stabilization of gaze and gait. It is also postulated that rotations of the body and head activate the angular vestibulo-collic and vestibulo-ocular reflexes (aVCR & aVOR) to provide stabilization of gaze for body movements around the yaw axis. Once normal gait is adequately described in this animal model, specific lesions will be made in the peripheral vestibular system and the effects of these lesions on straight and angular locomotion will be characterized. The vestibular lesions include canal plugging, canal-nerve section, unilateral labyrinthectomy and bilateral labyrinthectomy. It is postulated that these lesions will alter vestibular input and cause destabilization of gaze and posture as well as producing deficits in aligning the body to tilts in gravito-inertial acceleration that occur when running in a circular trajectory. Such control has obvious parallels in human locomotion and should provide specific information about the nature of vestibular control of gait and gaze in normal humans and in humans with vestibular lesions. To enable these studies, new technology has been developed that uses a head-fixed coil system to record eye movements in three dimensions in freely moving animals. Combined with precise angular and linear measurements of body, head and limbs in three dimensions, the normal movements of the body, head and eyes can be characterized and it will be possible to determine the effects of lesions of the vestibular apparatus on natural straight and circular locomotion of monkeys running in light and darkness.